The separation of solid particles from fluids is an important consideration in many industries from heavy construction to wastewater treatment. One industry in which the separation of solids from fluids is very important is rotary drilling. Rotary drilling techniques are used extensively in drilling hydrocarbon wells, water wells, and mining applications. In rotary drilling, a fluid, known as drilling mud, is circulated down a drill string, through a drill bit, and back up the borehole to the surface. The drilling mud acts as a lubricant for the drill bit and carries cuttings from the bottom of the borehole to the surface.
Most rotary drilling applications also rely on the drilling mud to control underground pressures developed by the formation fluids. Therefore, the density of the drilling mud is closely maintained in order to control the hydrostatic pressure that the mud exerts at the bottom of the well. If the mud is too light, formation fluids, which are at higher pressures than the hydrostatic pressure developed by the drilling mud, can enter the wellbore and flow uncontrolled to the surface, possibly causing a blowout. If the mud is too heavy, then the hydrostatic pressure excited at the bottom of the wellbore can reduce the rate at which the drill bit will drill the hole. Thus, the control of the solids content of the drilling fluid is very crucial to the overall efficiency and safe operation of the rig.
The cleaning and conditioning of drilling mud at the surface normally includes a series of solids control equipment, which may include shale shakers that remove large particles from the drilling mud, desilters and/or desanders that remove smaller particles, degassers that remove entrained gas, mud-gas separators to remove free gas, and dewatering equipment to reclaim drilling fluid while reducing hauloff. Clarifying tanks are also used in the cleaning and conditioning of drilling mud and, depending on their specific design, can be used for a variety of functions, such as large particle removal and fine solids dewatering.
One type of clarifying tank utilizes a cylindrical vessel into which drilling fluid is injected through a tangential nozzle, which causes the fluid to spin inside the tank. This spinning motion creates a vortex, which imparts centrifugal forces onto any solid particles within the fluid. These centrifugal forces move the particles away from the center of the tank, thus leaving a relatively clean fluid at the center. Any free gas will also migrate to the center of the vortex. A centrally located outlet near the top of the tank can then be utilized to withdraw the substantially clean fluid from the tank. The solid particles that are pushed to the outside of the circulating fluid are allowed to fall to the base of the tank where they can be collected and removed for recycling or disposal.
Some clarifying tanks have conical bottoms that collect solids and direct the particles toward a centrally located outlet where they can be removed from the tank. The outlet, typically located at the apex of the conical bottom, is often fitted with a valve that can be opened and closed to regulate the flow of solids exiting the tank. It is often desired to remove the collected solid particles from the tank without removing an excess of fluid. The valve can be used to control the characteristics of the material that is removed from the tank. One problem with some prior art configurations is that the opening and closing of the valve must be closely monitored to ensure the efficient removal of solids from the tank. If the valve remains open for too long, all of the accumulated solids may flow out of the tank and excess fluid will be lost through the outlet. If the valve is left closed for too long, the solids will build up in the bottom of the tank and may not flow once the valve is opened.
Thus, there remains a need in the art for methods and apparatus for separating solid material from a fluid. Therefore, the embodiments of the present invention are directed to methods and apparatus for separating solids that seek to overcome these and other limitations of the prior art.